1. Field of the Invention
The present invention relates to a vehicle operation assisting system for assisting an obstacle avoiding operation of a vehicle and a restoring operation following the avoiding operation.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 7-21500 discloses an automatic brake control system which determines the possibility of the subject vehicle on which the system is installed colliding with an obstacle based on a distance between the subject vehicle and the obstacle and a relative speed detected by the radar device; and automatically actuates a brake device of the vehicle when the possibility of collision is confirmed. In this system, when it is determined that collision with the obstacle cannot be avoided by only the braking force by the brake device and a steering operation of the driver which is detected, the turning property of the vehicle is enhanced by individually controlling braking forces of the left and right wheels and generating a yaw moment, thereby avoiding collision.
Also, Japanese Patent Application Laid-open No. 10-138894 discloses an automatic braking control system for avoiding collision by performing automatic braking when a radar device detects an obstacle ahead of the subject vehicle. In this system, when the driver performs a steering operation to avoid collision, a steering avoiding ability indicative of an ability of avoiding the obstacle is calculated based on the steering operation amount, and the magnitude of the braking force of automatic braking is controlled in accordance with the steering avoiding ability, thereby effectively securing the ability of avoiding an obstacle by the steering operation.
When a necessary target yaw rate to avoid collision with an obstacle ahead of the subject vehicle, which is detected by the radar device, is calculated, and yaw moment is generated by giving a braking force difference to the left and right wheels or by operating a power steering device, thereby performing a feedback control so that the actual yaw rate of the subject vehicle agrees with the aforesaid target yaw rate, there is a possibility that proper obstacle avoidance becomes difficult on a road having a low friction coefficient.
Namely, when the tire hardly skids on a road having a high friction coefficient, a yaw rate properly corresponding to the vehicle speed and the turning radius is detected, but when the wheels easily skid on a road having a small friction coefficient, there is a possibility that a yaw rate is detected because the vehicle rotates without properly turning, so that the angle of side slip of the tires increases. In the case where the vehicle rotates to generate the yaw rate in this way, if the feedback control is performed so that the yaw rate agrees with the target yaw rate, there arises a fear that the vehicle behavior cannot be properly controlled due to spin.